'Compound noun' is not in itself a traditional form class, but nevertheless a widely used and understood syntactic label.

Depending on the level to which one wishes to functionally label sentence constituents, a compound such as work permit can be analysed as realizing the combination [ADJ + NOUN], since 'work' (although canonically a noun) is here serving as a modifier to a noun (permit).

Alternatively, the labels adnominal + nominal would be preferred by some grammarians.